This invention relates to detecting the occurrence of physical contact between a conductive probe and a surface of a liquid held in a container, as the probe is lowered toward and eventually touches the liquid surface.
Such a probe may be used to remove a sample of a liquid, such as a biological specimen, from the container to perform testing. Typically, a number of sample containers, possibly representing different patients, are processed in sequence. Before the liquid can be retracted from a container, the system must ensure that the end of the probe is within the liquid volume since, if it is not, air, instead of the liquid sample, will be drawn into the probe.
Okawa et al., U.S. Pat. No. 4,736,638 ("Okawa"), detects when a pipette probe first touches a liquid in a container by capacitively coupling an AC signal via a conductive platter to the liquid in the container which rests above the platter. The magnitude of the resulting AC signal received by the probe as it is lowered into the container is measured as an indication of when the probe has touched the liquid surface. The signal magnitude is generally low until the probe contacts the liquid surface and then jumps to a higher level. Okawa's conductive platter is disk shaped to enable detecting when the probe contacts the liquid in any one of a number of containers arranged in a rack supported on the platter.